spline

Effect spl_x = spline(x / knotmethod=percentilelist(5 27.5 50 72.5 95) ); *Harrell*; 
Effect spl_x = spline(x / basis=bspline ); *B-spline (SAS default)*; 
Effect spl_x = spline(x / NaturalCubic ); *restricted cubic spline*;